Net Earnings Per Common Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Numerator
Net earnings available to common stockholders	$ 149.1	$ 255.9	$ 244.3
Distributed and allocated undistributed net earnings to non-vested restricted stockholders	(0.9)	(1.6)	(1.9)
Distributed and allocated undistributed net earnings to common stockholders	148.2	254.3	242.4
Distributed net earnings-dividends paid to common stockholders	(86.8)	(79.2)	(75.6)
Allocation of undistributed net earnings to common stockholders	61.4	175.1	166.8
Denominator
Weighted average number of common shares outstanding-basic	167.0	158.3	157.2
Basic net earnings per common share:
Distributed net earnings to common stockholders	$ 0.52	$ 0.50	$ 0.48
Allocated undistributed net earnings to common stockholders	$ 0.37	$ 1.11	$ 1.06
Basic net earnings per common share:	$ 0.89	$ 1.61	$ 1.54
Numerator
Distributed and allocated undistributed net earnings to common stockholders	148.2	254.3	242.4
Add: Allocated undistributed net earnings to non-vested restricted stockholders	0.4	1.1	1.3
Interest on 3% Convertible Senior Notes, net of taxes			4.4
Less: Undistributed net earnings reallocated to non-vested restricted stockholders	(0.3)	(1.0)	(1.2)
Net earnings available to common stockholders - diluted	$ 148.3	$ 254.4	$ 246.9
Denominator
Weighted average number of common shares outstanding-basic	167.0	158.3	157.2
Effect of assumed issuance of Settlement agreement shares	18.0	18.0	18.0
Effect of non-vested restricted stock units	0.4	0.4	0.2
Effect of conversion of 3% Convertible Senior Notes			7.2
Weighted average number of common shares outstanding-diluted	185.4	176.7	182.6
Diluted net earnings per common share	$ 0.80	$ 1.44	$ 1.35